Heroic: A Time Series Database from Spotify —
Heroic is an open-source monitoring system originally built at Spotify to address the problems being faced with large scale gathering and near real-time analysis of metrics.

Organize Complex Queries Using SQL's 'WITH' —
SQL:1999 introduced the ‘WITH’ clause to define “views” within queries. These with-queries are only visible in a single statement and can be used to improve the structure of this statement without polluting the global namespace.

HyperLogLog in Pure SQL —
Hyperloglog is a very interesting algorithm and you can have an extremely efficient version directly in Postgres. And if you’re not up for building and pulling in your own extensions here’s a raw SQL implementation of it.